The present invention relates to radiation-curable compositions useful as bonded ribbon matrices for optical fiber; to optical fiber ribbon arrays containing such matrices; and to processes for preparing such matrix-containing bonded ribbon arrays.
Optical glass fibers have revolutionized the telecommunications industry. The result has been a tremendous growth in demand for optical fibers which are free of many of the susceptibilities inherent to them.
Immediately after drawing, glass fibers are exceptionally strong and have very few intrinsic defects. However, such pristine fibers are very easily flawed by exposure to environmental conditions including dust and moisture. Therefore, there have been attempts to develop in the prior art numerous coatings which are capable of protecting the underlying glass fiber from external harmful forces and which optimally possess properties rendering them capable of obviating one or more of the various potential problems which may deleteriously effect optical fiber performance. Such properties include, inter alia, a glass transition temperature rendering the fiber useful over a large potential temperature use range; a higher refractive index than that of the fiber to refract any errant light signals away from the fiber; rapid cure, e.g., under ultraviolet irradiation; and high impermeability to moisture which may damage the coating or the fiber itself and may cause delamination of the two. Additionally, the adhesion level between the fiber and the coating must be optimized so that the coating will remain attached to the fiber during use but be easily stripped therefrom, with minimal damage to the integrity of the fiber and the coating, so that the fibers may be easily spliced in the field. Above all, the fiber coatings should display good thermal, oxidative and hydrolytic stability, to protect the underlying fiber over the long term, i.e., over twenty-five years' time.
In certain applications, such as in short haul, fiber-to-the-home uses, a single, coated optical fiber may adequately transmit a signal from one point to another. However, in most embodiments, a relatively large number of fibers are necessary to transmit a large volume of signals. For example, in the telecommunications industry, aggregates of fibers spanning oceans or continents and containing dozens of individual fibers may be required. Fibers are conveniently aggregated into cables, wherein large numbers of coated optical fibers are laid in parallel and are protected by a common sheathing material such as a layered arrangement which may include fiberglass, steel tape and reinforced rubber cabling material.
When numerous individual coated optical fibers are aggregated into a cable, it is necessary to be able to identify each of the individual fibers. For example, when two cable segments are to be spliced together, it is necessary to splice together ends of each like optical fiber in order for a signal to convey properly. When only a few fibers are contained in a cable, identification may be adequately made by having the coating of each individual fiber be a characteristic color; thus, the splicer may simply match up green fiber to green fiber, red to red, and so forth.
However, when the cable contains one hundred or more fibers, it may become impracticable to use a sufficient number of distinctive inks as to color each fiber distinguishably. Thus, a geometric means of distinguishing each fiber is used. For example, arranging the fibers in an array containing a number of layers, each layer containing perhaps twelve ink-coated fibers of different respective colors, will greatly facilitate the task of matching up fibers when splicing.
One practical way by which such spatial ordering of numerous fibers may be accomplished is to create a fiber array which is two-dimensional, wherein fibers are situated in a generally planar arrangement within a given array, with the fibers in the array disposed in parallelism with each other. These arrays are stacked one atop another in a three-dimensional structure.
Such arrays are known in the art as ribbons. For example, it is known to prepare a two-dimensional ribbon array by forming a "sandwich" of parallel coated optical fibers between two sheets of adhesive-coated Mylar tape, thus affixing the fibers in that configuration. This "sandwich" provides structural integrity and a tack free exterior surface.
However, this arrangement is less than optimal because the tape occupies a substantial proportion of the total volume of the sandwich, so that when several "sandwiches" are stacked to form a cable, an undesirably high proportion of the total cable volume is taken up by tape (rather than by optical fiber).
Thus, it has been envisioned to prepare an optical fiber ribbon array having a matrix in which the optical fibers are embedded in the desired generally planar, parallel arrangement. This matrix should, inter alia, have suitable glass transition temperature; cure rapidly; be non-yellowing; and have high thermal, oxidative and hydrolytic (moisture) stability.
However, currently available matrix materials possess a number of deficiencies which in the past have defied solution. First of all, it is a difficult if not paradoxical problem to devise a matrix composition which is adherent enough to maintain the integrity of an optical fiber array, yet not so strongly adherent that it will either remove some of the ink coating from the underlying coated and inked fiber when removed, or leave vestiges of itself upon the coated and inked fiber after it is stripped away. Removal of the ink from a coated, inked fiber is referred to in the industry as "breakout failure"; it can make identification of the once color-coded fibers difficult or impossible.
While matrix compositions are currently available which can be softened to leave an intact, inked coated optical fiber, these typically have the inconvenience of requiring application of a solvent (e.g., an alcohol gel), waiting at least ten minutes for the matrix to soften, and then peeling or scraping away the matrix. Beyond the convenience factor, vulnerability of the coated fiber to the solvent becomes a factor as well.
In other prior art situations, particulate release agents such as polytetrafluoroethylene (TEFLON.RTM.) particles have been either applied as separate coating layer to coated and inked fibers or incorporated into a matrix to confer strippability. However, use of particulates (which must be used in relatively large amounts) is highly undesirable because it makes application of the liquid composition difficult, and because settling of the particulate material in the liquid composition may occur. Also, stress concentration and other factors relating to the cured matrix composition which can be detrimental to the underlying fiber may result. Moreover, optical clarity may be compromised. Furthermore, in the case where the particulate is added as another coating layer, an additional, costly step is required to first treat the inked fiber with release agent.
In still other prior art situations, a complex, expensive mechanical stripping apparatus is required to remove the matrix composition. Use of such complex skill-requiring and expensive apparatus is, of course, undesirable.
Furthermore, it is a less than optimal situation if the matrix composition, even though eventually removable with its own and the ink layer's integrity intact, is removable in anything other than an intact unit. One can readily envision the inconvenience of peeling off bits and pieces of a matrix composition, not unlike removing from a roll a length of "scotch tape" which has split, broken or fragmented.